The present invention relates to a color image processing apparatus, and more particularly, to a color image processing apparatus which adjusts parameters for image processing, using a patch image.
Image forming apparatuses, such as copying machines and printers, have recently been adapted to mainly do color printing. Accordingly, print color is required to have precisely specified gradation characteristics. For example, a copying machine performs color conversion to convert RGB image data input from a scanner to CMY image data to be output on a printer using color material. To duplicate an image so that the original tone of color is faithfully reproduced, color conversion characteristics (color conversion parameters and a color conversion table) must be adjusted properly.
To duplicate a color image faithfully, gradation characteristics must also be adjusted properly. When passing through a sensor or a filter, image data input from a scanner suffers distortion for R, G, and B. When a printer is provided with the same image data, images which differ in gradation and color are output from time to time due to different environments (different temperatures and humidities), time-dependent changes, and differences between individual products. Especially because the gradation characteristics of a printer sensitively change in response to various factors, a mechanism which properly adjusts the output gradation characteristics and color conversion characteristics of the printer must be installed in the printer to keep reproducing an image well.
A method for determining color conversion parameters is known which collects multicolor patch RGB-CMY data pairs by making an apparatus to read a color patch sample output therefrom and finds by the least-square method the coefficient of each term of a polynomial for converting RGB data to CMY data. The method, which considers image processor characteristics other than color conversion to be like a black box, is based on the idea that ideally, an image can be reproduced faithfully only by color conversion, because black box inverse corrections are made using color conversion.
A method is also known which color conversion parameter setting are fixed under an assumption and adjusts output gradation characteristics in the same way as described above. That is, the method collects multigradation patch C′, −C, M′, −M, Y′, and −Y data pairs by making an apparatus to read a color patch sample output therefrom and finds by the least-square method the coefficient of each term of gradation correction equations for C, M, and Y.
However, it is difficult to output an image with satisfactorily reproduced gradation and color even using the above-described adjusting methods. This is because image processing performed by equipment from a scanner to a printer comprises the steps of color conversion, blackening, gradation processing (quasi intermediate gradation processing), screen processing, etc. in which image data suffers nonlinear distortion with different characteristics.
Thus a problem with conventional image forming apparatuses it that it is difficult to generate all parameters at a time which are used for correcting nonlinear distortion due to color conversion, blackening, gradation processing, screen processing, etc.
Another problem with conventional equipment is that it is difficult to correctly estimate record gradation for characteristics of individual equipment and incorporate the estimated record gradation into parameters for processing of individual signals.